Dating the Past:
Today, that a particular rock is a million ycars old, or a fossil is 50 thousand ycars old. For instance, how do we estimate the duration of various geological periods on a time scale of billions of years? Initially, such estimates were based on the rate at which geological processes such as depositions of stones and rocks occur. Many layers of rock originated as deposits in the sca~and at the mouths of rivers. Therefore. one way to get an idea of the length of time required to produce a deposit of a given thickness, is to measure the rate at which rivers arc depositing sediments in thc sca today. With the dcvcloping knowlcdgc of radioactivity. more accurate dating of fossils and certain types of rocks has become possible. Radioactive substances can be easily detected using certain instruments. They have built-in "clocks" in the form of 'radioactive isotopes' that change or decay at a constant rate into non-radioactive form. If this rate is known, the length of time since the fossil or the rock was formed can be estimated by measuring the quantities in the rock, of the radioactive isotopes arid the non-radioactive ones into which they have changed. For example, Uranium is transformed into certain isotopes of lead which are not radioactive. So, the age of uranium-containing rocks can be determined by comparison of the proportions of undecayed Uranium and that of the corresponding lead isotopes present in the rock. see Fig. The method most commonly used, now, for estimating the age of fossils is radio-carbon dating. Radioactive isotope of carbon is commonly known as carbon- 14. Since carbon- 14 is chemically the same as ordinary carbon, both arc absorbed by plant and animal tissues in the same proportion as they arc present in the atmosphere as carbor. dioxide. Plants use this carbon dioxide in making their food. Animals eat the plants. Hence, the proportion of carbon- 14 in the tissues of plants and animals is the same as in the atmosphere, as long as the plant or the animal is living. But as soon as it dies, no
more carbon can enter its body as photosynthesis or food intake stops. Following death, the carbon-14, already present in the body, decays steadily into ordinary carbon. So the smaller the number of carbon-14 atoms remaining, the older is the fossil. Thus, if we take a piece of ancient wood or bone and measure the amount of carbon-14 present in it, we can estimate the age of the material. This technique has been applied to materials of known age, and thus its accuracy was tested giving confidence in the age determinations of unknown samples. The carbon-14 method is applicable only to organic materials which still contain carbon. It cannot be used for fossils in which all organic matter has decayed. In that case the age of the fossil can be estimated by determining the presence of other radioactive elements like flourine, or phosphorus.